DSP
A few models include built-in digital signal processing: compression, limiting, EQ, filtering, and so on. The advantages are:
• There is less gear to lug around in racks.
• EQ and limiting presets can be set up in DSP to work with specific loudspeaker models. Just select a preset that works with your chosen speakers.
• Some DSP includes diagnostics such as load monitoring to check for blown speakers, error logging, and so on.
Networking
Another feature in many modern amps is networking. Network-capable amps can be part of an interconnected audio network, so they can be controlled and monitored from a central computer. This beats walking around on stage trying to figure out which amp has shut down.
Amplifier Class
Let’s turn now to another aspect of power amplifier design. Amplifier class refers to the circuit design of the output stage, such as Class A, Class AB, Class D, and so on.
As a background for this section, remember than an audio signal has a positive half of the cycle and a negative half of the cycle (Figure 2).
Transistors are basically rectifiers; they can conduct (pass current) only during the positive or negative half unless they are biased by a certain amount. The bias can create a DC offset in the signal.
Here are the features of the most common classes:
Class A
• Has enough bias (DC offset) to shift all of the audio signal into the positive region in the output devices (Figure 3). As a result, positive/negative signal halves become more positive/less positive changes.
• Because the output transistors are always on, current flows at all times. This design generates a lot of heat. Some power is dissipated even when there is no music playing.
• Lowest distortion.
• Least efficient (typically 20 percent); wastes a lot of energy.
• Typically used in audiophile applications up to 300 W per channel.
Class B
• The output devices are in push-pull pairs: one device amplifies the positive half of the sine wave signal, and the other amplifies the negative half. Each device of the pair is on for half of the signal cycle (positive or negative voltage) and off for the other half of the cycle. Each device conducts for a half cycle.
• Much more efficient than Class A (typically 60-70 percent).
• Less heat.
• There is discontinuity at the transition point between transistor signals near 0 volts (Figure 4). This results in high “crossover distortion” or “switching distortion” – low sound quality.
• Typically used for pocket radios or clock radios.
Class AB
• Both output devices in each pair are biased slightly on which reduces crossover distortion. Each transistor operates slightly more than half the cycle but is off for a fairly long time, which reduces heat dissipation. (Figure 5).
• About 50 percent efficiency.
• Low distortion.
• Typically used for home stereos and pro audio amps up to 600 W per channel.
Class G or H
• Both of these classes include two DC rails (DC supplies) of low voltage and high voltage. The high-voltage rail is switched on only when the input signal demands it, which reduces the amount of heat in the output devices. The power supply is signal-controlled.
• In Class G, one output stage is fed by the low voltage rails and another stage is fed by the high-voltage rails. The low-voltage stage is always on, and the high-voltage stage turns on only when the signal exceeds a threshold level. Class H uses only one output device stage which is fed variable supply voltages depending on the amplitude of the signal.
• Fewer output devices and less heat sinking are required, which reduces the weight and size of the amp.
• Tends to have elevated distortion at high frequencies due to the switching.
• Typically used for LF or MF applications from 400 to 3000 W per channel.
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